Liquid detergents have hitherto been used mainly for light duty applications such as dish washing. The market for heavy duty detergents, e.g. laundry detergents, has been dominated by powders, due to the difficulty of getting an effective amount of surfactant and in particular of Builder into a stable liquid formulation. Such liquids should in theory be cheaper than powder detergents since they would avoid the need to dry and would in many instances replace the sulphate filler conventionally used in powder detergents with water. They also offer the possibilities of greater convenience and more rapid dissolution in wash water than powder. Attempts to provide solutions of the Functional Ingredients have been relatively unsuccessful commercially. One reason for this lack of success has been that the most commonly used and cost effective Builders e.g. sodium tripolyphosphate are insufficiently soluble in aqueous compositions. Moreover due to salting out effects, increasing the amount of dissolved Builder tends to lower the amount of surfactant that can be dissolved and vice versa. Potassium pyrophosphate Builders, together with amine salts of the Active Ingredients, which are more soluble, have been tried as alternatives to the sodium salts but have not been found cost effective.
Unbuilt liquid detergents containing high levels of surfactant have been marketed for laundry use, but are unsuitable for hard water areas and have enjoyed only limited success, mainly confined to markets where the use of effective builders is subject to legal constraints, and the competition from powders is correspondingly less severe.
A different approach is to attempt to suspend the excess Builder as a solid in a liquid micellar solution, or emulsion, of surfactant. The problem however has been to stabilise the system to maintain the Builder in suspension and prevent sedimentation. The literature has proposed numerous, relatively sophisticated formulations, including the use of expensive potassium salts in place of cheaper sodium salts and of solubilisers such as Hydrotropes, dispersants or solvents, all of which have prevented realisation of the potential cost saving. Even with such extraneous additives it was considered necessary to use relatively low concentrations of solid Builder, giving limited washing effectiveness. This approach has been conditioned by certain assumptions: that the Active Ingredient should as far as possible be in solution; that the amount of Active Ingredient should be relatively high; that the amount of suspended solid should be minimised to avoid difficulties in stabilising the suspension against sedimentation; that special thickeners or stabilisers were essential to prevent sedimentation; and that Electrolytes which would desolubilise the surfactant be omitted or kept at very low levels.
A major feature of the art hitherto has been its empyrical nature. No acceptable general theory has been proposed to account for the stability of some compositions and the instability of others. There is thus no way of predicting which compositions will be stable, and no general procedure for designing a novel Stable liquid detergent. The art contains no generally applicable teaching, and even the specific examples of most of the prior patents on liquid detergents, provide compositions which separate within a few weeks. The relatively few exceptions have been apparently discovered by chance and no extrapolation has been possible.
Products of this type have been introduced commercially in Europe and Australia, but have suffered from certain serious drawbacks. The products have been relatively poor washing performers, either due to low Builder:Active Ingredient weight ratio, or to low alkalinity. They have also shown evidence of undesirable sensitivity to mechanical and/or thermal stress, e.g. on shearing or storing under extreme climatic temperature conditions. Thus some compositions separate on shearing, others become excessively viscous. Most separate after storage at either 0.degree. or 40.degree. C. However the art has not taught how to overcome the foregoing shortcomings.
In addition to the compositions which have been developed commercially, many compositions have been proposed in the literature which are not suitable for commercial exploitation in practice. Typically such compositions are unstable, or insufficiently stable to withstand normal storage without undergoing sedimentation, or else are too expensive to manufacture in relation to their washing effectiveness, to be considered for commercial development.
There has recently been proposed a set of compositions in which the Active Ingredients form a network of a Lamellar Phase, separable from the aqueous phase by Centrifuging, which provides a gel structure capable of supporting suspended particles of solid Builder. The gel structure is obtained by addition of sufficient Electrolyte to salt out the Active Ingredient, so as to form an aqueous Lye phase and a Separable Lamellar Phase, and by maintaining the solids content above a threshold for stability and below a ceiling for Pourability. The amount of Electrolyte required depends upon the hydrophilicity and melting point of the surfactant, and whether any solubilising additives such as Hydrotropes or solvents are present. The aforesaid gel compositions tend to be higher in Payload and in the ratio of Builder/Active Ingredient, and to be more cost effective than known commercial liquid compositions. Indeed, the best of the aforesaid Lamellar gel compositions are most cost effective soil removing agents than the best laundry powders.
However, the Lamellar Compositions disclosed hitherto exhibit a mobility which is lower than is desirable for some purposes.